Geoscience Reference
In-Depth Information
rain down through the water column. h e microscopic fossils allowed geolo-
gists to derive the pattern of the ice ages for the past two million years.
Pioneering research measuring the oxygen isotopes in tiny fossil shells
was conducted by Cesare Emiliani in the laboratory of his mentor, Harold
Urey, at the University of Chicago. Emiliani developed the use of oxygen iso-
topes in paleoclimate research—a technique whose value, as we have already
seen, continues to this day. It is based on the more rapid evaporation of the
lighter isotope of oxygen (oxygen-16) from the ocean than the heavy isotope
(ox yg en-18). h is evaporated water vapor is carried to higher latitudes and
continents, some precipitating as snow on land, building great ice sheets dur-
ing glacial periods. As the oxygen-16-rich water is removed from the oceans
and stored as ice on land, sea level is lowered, and the ocean water becomes
more enriched in the heavy isotope of oxygen (oxygen-18). When marine
organisms (such as the microscopic foraminifera) build their shells of calcium
carbonate, they incorporate this oxygen, recording changes in sea level and
glacier growth on land.
Emiliani published a landmark paper in 1957 that described oscilla-
tions every 100,000 years between two opposing climate states: cold glacial
periods, lasting up to 90,000 years, and warmer interglacial periods, last-
ing only about 10,000 years. h e temperature dif erence between the two
states is approximately 6°C (11°F) or more. h e shit from the warmer to the
cooler state into an ice age is relatively slow, as ice builds up on the Northern
Hemisphere continents over the course of tens of thousands of years. In
contrast, the shit from the cooler to the warmer state (from glacial to inter-
glacial) takes only thousands of years. h is pattern of climate change over
the past million years has been described as “sawtooth” because of its sharp
up-and-down shape when temperature or sea level is plotted against time.
evidence from ice cores
Ice sheets on Greenland and Antarctica contain in their layers of ice detailed
information about past global temperatures, atmospheric composition,
windiness, and more. Over the past two decades, cores extracted from these
enormous ice sheets have been used to reconstruct past climate. In one proj-
ect, researchers extracted atmospheric gases from bubbles trapped in the
ice to measure past levels of carbon dioxide. h eir results show that carbon
dioxide levels in the atmosphere varied with temperature during the ice ages
